This invention relates to a thermophotovoltaic (TPV) apparatus for generating electricity using a sequence of selective emitter zones, each zone producing at least one spectral band matched to the absorption bands of a photovoltaic device. The highest temperature zone consists of an emitter with the highest energy photon emissions, and the second zone with the next highest energy and so on to produce a sequence of emitters that can remove more energy from the heat source than a single emitter system. The increase in efficiency results because the temperature drop of the multi-zone emitter system is greater than the temperature drop over any single emitter system.
Electric power can be generated by heating any photon emitter to a sufficient temperature that it will emit photons above the band gap of the PV. Such photons are absorbed by photovoltaic devices, such as a silicon cell, to produce an output voltage and current. In this novel invention a series of thermally stimulated quantum emitters, each of which produces radiation in a relatively narrow spectral band when heated above a threshold temperature, are matched to a corresponding PV cell. A variety of photovoltaic devices are commercially available for absorbing such radiation. In this invention these emitters are sequenced by zone to match the band gap of PV cell with the selected emissions such that the higher temperature, higher band gaps are closest to the heat source. Then the next highest and so on to produce a TPV system that is very efficient, because it can remove more energy from the heat source.
U.S. Pat. No. 3,188,836 by Kniebes describes utilization of emissive radiation from a glowing mantle in a gas lamp to generate sufficient power to control a valve. U.S. Pat. No. 3,331,701 by Werth provides a description of a thermophotovoltaic power producing device. R. M. Swanson in "Silicon Photovoltaic Cell in TPV Conversion," ER 1272, Project 790-2, Stanford Univ., (December 1979), who pioneered the fundamentals of blackbody thermophotovoltaic devices describes an efficient solar cell. This cell work was initiated to optimize the performance of silicon cells when used in conjunction with a blackbody emitter. Swanson has also reported that these cells produce electric power with an efficiency of 26% using a tungsten filament heated to about 2200.degree. K. as the heat source. Proceedings of IEEE 67 (1979) 446;ER-1277, Project 790-2, Stanford Univ., (December 1979).
Narrow band emitters are known to be more efficient power generators than blackbody emitters, operating at the same input power level, that generate power by radiation. This is disclosed, e.g., in U.S. Pat. No. 4,973,799. In addition, European Patent Publication 84 306033.6 by Nelson, and European Patent Publication 83 108018.9 by Diederich disclose advantages of using rare earth metal oxide narrow band emitters matched to the absorption characteristics of photovoltaic devices. Goldstein also produce efficient energy conversion for use in powering various gas appliance in U.S. Pat. No. 4,906,178 as well in power production applications.
British Patent Number 124 by Carl Auer von Welsbach was the origin of the first successful gas light mantle almost 100 years ago. That structure comprises a thermally stimulated quantum emitter comprising ceria. Such a mantle is designed to emit a broad band spectrum of white light rather than a narrow band. In "High Temperature Spectral Emitters of Oxides of Erbium, Samarium, Neodymium, & Ytterbium" Applied Spectroscopy, 26 (1972) 60-65, Guido Guazzoni suggested the use of narrow band emitters for electric power production. His data suggested that the spectral emittance of ytterbia (Yb.sub.2 O.sub.3) is particularly well suited for use with silicon photovoltaic devices in a power production system.
In the late 1970s and early 1980s there was appreciable research on blackbody thermophotovoltaic devices reported in R. N. Bracewell and R. M. Swanson, "Silicon Photovoltaic Cells in TPV Conversion," EPRI ER-633, (February 1978); J. C. Bass, N. B. Elsner, R. J. Meyer, P. H. Miller, Jr., and M. T. Sinmad, "Nuclear-Thermophotovoltaic Energy Conversion," NASA CR-167988 (GA-A16653) G. A. Technologies, Inc., (December 1983); L. D. Woolf, J. C. Bass, and N. B. Elsner, "Variable Band Gap Materials for Thermophotovoltaic Generators," GA-A18140, GA Technologies, Inc., (December 1983); and papers mentioned above. In these documents, Swanson indicates that efficiencies of at least 50% may be possible and he has measured photon conversion efficiencies of about 30% with a relatively crude experimental setup using a blackbody emitter. Fahrenbruch states that the photovoltaic conversion efficiency when using an emitter which emits narrow band radiation, may be much greater than that obtained using blackbody radiation.
The reason for this improvement in conversion efficiency is that the energy required to promote an electron from the conduction band to the valance band is equivalent to a specific quantity of energy or wavelength, the band gap energy. For each photon absorbed by the photovoltaic device, one electron is promoted into the conduction band. If the photons absorbed have energy in excess of the band gap energy, the excess energy is converted into heat or phonons and this decreases the conversion efficiency. It is, therefore, desirable to absorb radiation with minimal deviation from the band gap energy.
The next major steps in the development of thermophotovoltaic power technology involved improvements in materials science. In European Patent Publication 84 306033.6, R. E. Nelson describes a small strong mantle capable of withstanding 1000 g. This is almost 100 times stronger than the Welsbach type mantle. In U.S. patent application Ser. No. 07/864,088, filed May 16, 1986, now abandoned Goldstein and Goldstein et al show several approaches to improve selective emitters (U.S. Pat. Nos. 4,776,895, 4,793,779, 4,806,095, 4,906,178, 5,281,131, 4,898,531 and 5,356,487). In a totally new approach for strong emissive devices for implementing both small and large scale power generation appliances described by Goldstein in U.S. Pat. No. 5,503,685, and U.S. patent application Ser. No. 08/370,963. K. C. Chen, in a patent application that is to be filed, has described yet another major step in strength improvement of TPV selective emitters. Eva Wong has described, in U.S. Pat. No. 5,837,011, a method of manufacturing a ceramic felt that was employed in the device that demonstrated 2.4 Kwe, a world TPV power record as of April 1995 (Holmquist et al).
This multi-zone superemissive invention allows one to produce a series of improved TPV devices for the self-powering of various appliances to TPV electric power generators such as were originally described in U.S. Pat. Nos. 4,906,178, 5,356,487, 5,281,131 and several patent applications mentioned above; and the improvements over the originals are further described herein.
Ytterbia is a narrow band emitter which emits photons over a narrow range of energies with a half band width of about 100 nanometers centered at about 950 to 1000 nm. Use of this emitter material produces a substantial improvement in the thermophotovoltaic energy conversion efficiency when compared to the use of blackbody emitters and leads to the design and development of many practical devices for generation of electric power. As shown by Nelson in U.S. Pat. No. 4,584,426, the emissive output over the range of from 400 to 2500 nanometers should have 50% of the radiant energy within a single band.
Chubb estimated the optimum emitter temperature for a maximum efficiency for ytterbia superemitters of 3000.degree. K., for erbia of 2000.degree. K., for hohmia of 1500.degree. K., and for neodimia of 1450.degree. K. (Ref). It means that by placing the different superemitters in various temperature environment we can create such temperature profile that each emitter will generate narrow band photon emissions with the maximum efficiency.
It is clearly of significance in power generating system to enhance the thermal to light conversion efficiency. Thus, it is desirable to enhance the proportion of the thermal energy that goes into heating the emitter which is finally absorbed by the photovoltaic devices and efficiently converted to electric power.
It is also clearly advantageous to be able to create photons within a optical system that can concentrate the photons and or direct them to various target such as lasers, photovoltaic device, photolithography, photochemical reactors, photobiological reactors, photophysical reactors, plants, and other applications where photons are desired.
It is also clearly important to provide an effective PV cell temperature management as well as realize incoming combustible mixture preheating due to waste heat recuperation from emitters (radiant energy) and flux gases (convective heat).